Along with rapid diffusion of wireless devices typified by cell phones, it is growing a demand for a compact, lightweight resonator and a filter that combines the resonators. For this demand, a piezoelectric thin-film resonator have attracted attentions, while a dielectric filter and a surface acoustic wave (SAW) filter have been mainly used. Because the piezoelectric thin-film resonator is a device with an excellent characteristic relative to, especially, a high frequency, which can be downsized and monolithically integrated.
The piezoelectric thin-film resonator may be classified in such as a Film Bulk Acoustic Resonator (FBAR) and a Solidly Mounted Resonator (SMR). The FBAR is configured by laminating a lower electrode, a piezoelectric membrane, and an upper electrode on a substrate. A cavity space is formed below a portion where the lower electrode and the upper electrode oppose each other across the piezoelectric membrane which resonates, where the portion is called as a membrane region.
Here, two types of the cavity space in the FBAR are known, one is a cavity space (cavity) formed between the membrane region and the other is a cavity space (via hole) formed through the substrate below the membrane region.
The via hole is formed in the substrate through wet etching, dry etching, or the like. An FBAR having a via hole (hereinafter referred to as “via hole type FBAR”) is manufactured based on, for example, a substrate back side Via process.
One example of a manufacturing method for an FBAR having a cavity (hereinafter referred to as “cavity type FBAR”) is a substrate surface processing method, which is shown in Japanese Laid-open Patent Publication No. 2000-69594 for example. The other example is an air bridge method, which is shown in Japanese Laid-open Patent Publication No. 2006-211296 and No. 2007-208728 for example. Either method can form a cavity by wet-etching a sacrifice layer formed between a substrate surface and a membrane region.
In the methods, for example, a sacrifice material is formed in a region for forming a cavity in advance, and then a lower electrode a piezoelectric membrane, and an upper electrode are formed thereon, where the electrodes and the piezoelectric membrane mainly constitute a piezoelectric thin-film resonator. After the lamination of these layer, a through-hole is formed to reach the sacrifice layer from an upper surface of the piezoelectric thin-film resonator, and the sacrifice layer is removed by executing etching through the through-hole.
Since the manufacturing process of the cavity type FBAR needs no process forming the cavity space from the rear side (back side) of the substrate, the cavity type FBAR is more suitable for mass-production than the via hole type FBAR form the viewpoint of a process for dicing a chip to handle each diced chip.
As shown above, the cavity type FBAR needs a portion on a surface of the device for an inlet and flow channel of an etchant for removing a sacrifice layer. The portion becomes a dead space not functioning as a resonator. In particular, the dead space is a big barrier to size reduction in a ladder type filter configured by connecting plural FBARs with each other.
As a countermeasure against this problem, proposed is a method for forming an inlet of an etchant for removing a sacrifice layer on the rear side (back side) of the substrate, which is shown for example in Japanese Laid-open Patent Publication 2005-333642.